In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters and images. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using a plain paper as the recording medium, non-contact with printed images and characters, etc.
Among such printing methods, in view of enhancing the weather resistance and water resistance of printed images and characters, an ink-jet printing method utilizing an ink containing a pigment as the colorant has now come to dominate.
JP 10-292143A discloses a water-based ink used in a thermal printing method of injecting an ink by the action of bubbles generated upon heating the ink, which includes a water dispersion containing a colored microcapsule obtained by covering a pigment with a film-forming resin. However, the film-forming resin component is a water-soluble polymer or a self-dispersible polymer and, therefore, is dissolved in the ink, thereby failing to allow the ink to exhibit a satisfactory ejecting property.
JP 2003-138176A discloses a method of improving an ejecting property of a water-based ink containing a water dispersion of colorant-containing polymer particles by subjecting the water-based ink to membrane treatment or centrifugal separation treatment to thereby remove a water-soluble polymer and a self-dispersible polymer from the water dispersion. However, this method has failed to attain satisfactory purification efficiency and productivity.
JP 2006-152270A discloses a method of purifying a colorant-containing water-insoluble polymer with a solvent having a specific solubility parameter to improve an optical density and an anti-bleeding property of an ink. However, the ink obtained by this method tends to be unsatisfactory in ejecting property.
In recent years, with a highly precise structure of a printing head, a diameter of nozzles provided therein tends to be reduced, and besides with the increase in printing speed, there is an increasing demand for further improving an ejecting property of an ink.